PRESIDENT Reagan says that at Reykjavik, Mikhail Gorbachev demanded too much as a price for an agreement on reductions in armaments - a veto over the development and deployment of the Strategic Defense Initiative (SDI, or ``star wars''). That may sound like an unrealistic demand, but is it really, when Mr. Gorbachev already has a veto? He has it by virtue of the fact that he could, and would, physically obstruct us from building a space-based defense against ballistic missiles. To understand that, let's put the shoe on the other foot. What if United States intelligence told us that the Soviets were on the verge of constructing an SDI of their own invention? We would have to be very concerned that our ballistic missile force might not be able to retaliate if we were attacked. If the Soviets also began multiplying air defenses to the point that we would not be certain that our bombers and cruise missiles could get through, we might conclude that we were about to lose our ability to retaliate at all to a Soviet nuclear attack. The doctrine of mutual assured destruction (MAD), which has restrained both us and the Soviets from initiating nuclear war for more than 30 years, would have broken down.
That could mean the Soviets might initiate a nuclear war on the presumption we could not strike back. Or, more likely, they might blackmail us by threatening a nuclear attack including, perhaps, a demonstrative explosion in some remote area of the US. In the first case, our existence would be at stake. In the other, our free way of life would be in jeopardy. We'd be close to desperate.
What would we do? There is no question in my mind. We would attack. We'd not attack the Soviet Union, for it would almost certainly respond with a devastating nuclear attack on the US. Rather, we'd attack its SDI the moment it began to construct it. When the first building blocks went up into space, we'd shoot them down, using either our antisatellite systems or simply a nuclear explosion in the vicinity of the Soviets' space stations. We'd have to do it before they had sufficient components in place to defend themselves.
How would the Soviets respond? They might knock down some of our space activities, like photographic satellites. Or they might complain in the United Nations. Or they might break relations with us, but they would not attack our soil with nuclear weapons. They would still be deterred by our assured retaliatory capabilities. MAD would prevail.
What, then, if the shoe were on the Soviet foot instead? It is almost certain they would do the same, that is knock out our SDI. Our scientists estimate that it will take some 600 launches into space over two years to build an SDI. Even though we might be able to build an increment that could defend itself in less time, there will be a period of nearly total vulnerability.
So the Soviets have a veto over our deploying an SDI even now. What does that mean in terms of how we negotiate in the wake of Reykjavik? The President has staked out his position that after 10 years of further observance of the ABM Treaty, we would be free to proceed to SDI. Gorbachev has staked out his position that we could not proceed without Soviet concurrence - a veto. Clearly he would prefer a negotiated veto to one he'd have to enforce by shooting. Here, though, is where our negotiators need to recognize that Gorbachev can fall back on his shooting veto if he cannot get a written one.
Our negotiators should also understand there are real advantages for us in acknowledging the Soviet veto. We have lived for more than 30 years in a world of only offensive nuclear forces and strategies. The balance that has kept each side from using offensive forces has been delicate, so delicate that it would be very risky for either to shift the rules of the game unilaterally to a strategy of defense. Such a dramatic move can be made only in tandem without a real danger of misunderstanding and overreaction.
Ironically also, the best hope for the President's dream of a nuclear world in which defenses predominate is to reduce the size of both nuclear arsenals substantially. Building a strategic defense for our entire country against the size of the nuclear arsenal the Soviets now have, let alone could build, is clearly a formidable undertaking. It's probably not feasible. There may, though, be hope for building a defense if the threat is much smaller.
We will get such reductions in the Soviets' threat only through negotiated agreements. To induce them in that direction, we would have to agree we would not deploy an SDI without their concurrence. Otherwise, their natural inclination would be to proliferate weapons to give them a hope of being able to overwhelm our SDI if, for any reason, they did not, or could not, shoot it down.
The fact that SDI became a cause cel`ebre at Reykjavik has opened the door to much-needed discussion of the interrelationships between offense and defense in the world of nuclear weaponry. The heart of that relationship is that today's offensive weapons will permit either side to prevent the other from building a space-based defensive system. That's very good in the search for nuclear stability.
Stansfield Turner, author of ``Secrecy and Democracy,'' is a former director of central intelligence.